Sputtering system and target electrode construction for use therewith

ABSTRACT

SPUTTERING SYSTM HVING A HOUSING FORMING AN ENCLOSED CHAMBER WITH MEANS FOR SUPPLYING A VACUUM TO THE CHAMBER IN THE HOUSING. SUPPORTING MEANS IS PROVIDED WITHIN THE CHAMBER FOR SUPPORTING MEANS IS PROVIDED THE HOUSING IS PROVIDED WITH A SUPPORT PLATE HAVING AN OPENING THEREIN WHICH IS GENERALLY OPPOSITE THE SUPPORT MEANS. A TARGET ELECTRODE IS DISPOSED IN OPENING AND FACES THE SUPPORT MEANS. AN INSULATING MEMBER IS SECURED TO THE TARGET ELECTRODE AND TO THE SUPPORT PLATE TO SUPPORT TH TARGET ELECTRODE SO THAT IT IS SPACED FROM THE SIDE WALL OF THE SUPPORT PLATE FORMING THE OPENING AND HAS ITS INNERMOST SURFACE GENERALLY FLUSH WITH THE INNERMOST SURFACE OF THE SUPPORT PLATE AND SO THAT THE SPACE BETWEEN THE EDGE OF THE TARGET ELCTRODE AND TH WALL OF THE SUPPORT PLATE DEFINING TH OPENING IS LESS THAN ONE DEBYE LENGTH. MEANS IS PROVIDED FOR SUPPLYING A VOLTAGE TO THE TARGET ELECTRODE.   D R A W I N G

June 26, 1973 URBANEK ET AL 3,741,886

SPUTTERING SYSTEM AND TARGET ELECTRODE CONSTRUCTION FOR USE THEREWITHFiled June 28, 1971 2 Sheets-Sheet 2 254ml Uzsn/vek p Ease/v G ksesw/Lz.

I N VEN TORS 44,, M M M- United States Patent 3,741,886 SPUTTERINGSYSTEM AND TARGET ELECTRODE CONSTRUCTION FOR USE THEREWITH KarelUrbanek, Atherton, and Edson G. Kerswill,

Mountain View, Calif., assignors to Randex, Inc., Palo Alto, Calif.

Filed June 28, 1971, Ser. No. 157,419 Int. Cl. C23c 15/00 US. Cl.204--298 16 Claims ABSTRACT OF THE DISCLOSURE Sputtering system having ahousing forming an enclosed chamber with means for supplying a vacuum tothe chamber in the housing. Supporting means is provided within thechamber for supporting an article to be coated. The housing is providedwith a support plate having an opening therein which is generallyopposite the support means. A target electrode is disposed in theopening and faces the support means. An insulating member is secured tothe target electrode and to the support plate to support the targetelectrode so that it is spaced from the side wall of the support plateforming the opening and has its innermost surface generally flush withthe innermost surface of the support plate and so that the space betweenthe edge of the target electrode and the wall of the support platedefining the opening is less than one Debye length. Means is providedfor supplying a voltage to the target electrode.

BACKGROUND OF THE INVENTION This invention relates to sputtering systemsand more particularly to the construction of the target electrode forsuch sputtering systems.

In sputtering systems provided in the past, the electrodes utilized insuch sputtering systems have had a very high electrical capacitance toground. This has been found to be particularly troublesome when RFsputtering is utilized. A large electrical capacitance to ground leadsto excessive circulating currents to and from the electrode to which theradio frequency energy is supplied. In order to prevent the leads fromburning up, it has been necessary to make such leads inordinately large.Also, in prior art sputtering systems, in order to prevent glowdischarge from the electrode to which the RF energy is supplied, it hasbeen necessary to provide a shield which is closer to the electrode thanone Debye length which also greatly increases the capacitance betweenthe electrode and ground. There is, therefore, a need for a new andimproved sputtering system and target electrode construction for usetherewith which overcomes such difliculties.

SUMMARY OF THE INVENTION AND OBJECTS The sputtering system consists of ahousing which forms an enclosed chamber. Means is provided for supplyinga vacuum to the chamber. Supporting means is provided within the chamberfor supporting an article to be coated. The housing includes a supportplate which has an opening therein generally opposite the support means.

A target electrode formed of a conducting material is then provided. Aninsulating member is secured to the target electrode and to the supportplate and serves to support the target electrode in said opening so thatthe target electrode faces the support means within the housing and sothat the edge of the support plate is spaced from the Wall defining theopening in the support plate by a space which is less than one Debyelength and so that the innermost surface of the target electrode isgenerally flush with the innermost surface of the support plate.

In general, it is an object of the present invention to provide asputtering system with a target electrode in which the electricalcapacitance to ground is reduced significantly.

Another object of the invention is to provide a sputtering system inwhich only inert gas is utilized and in which only a portion of thetarget electrode is exposed to the inert gas.

Another object of the invention is to provide a sputtering system of theabove character in which the target electrode can be readily cooled.

Another object of the invention is to provide a sputtering system of theabove character in which the target electrode has its side edge and backside shielded.

Another object of the invention is to provide a sputtering system of theabove character which can be utilized for depositing a wide variety ofmaterials on the substrates.

Another object of the invention is to provide a sputtering system of theabove character in which a dark space shield is provided.

Additional objects and features of the invention will appear from thefollowing description in which the preferred embodiment is set forth indetail in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-sectional view withcertain parts broken away of a sputtering system incorporating thepresent invention.

FIG. 2 is an enlarged partial isometric view of the area encompassed bythe line 2-2 in FIG. 1 and in particular shows the construction of thetarget electrode assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT The sputtering systemincorporating the present invention is comprised of a housing 11. Thehousing 11 consists of a cylinder 12 formed of a suitable material suchas glass. The cylinder 12 is open at the bottom and top ends which areadapated to be closed by top plate 13 and a bottom or base plate 14.Gaskets 16 and 17 are provided for establishing an air-tight sealbetween the cylinder 12 and the top and bottom plates 13 and 14 so thata vacuum chamber 18 is formed within the housing 11. Means is providedfor supplying a vacuum to the chamber 18 and consists of a vacuumpumping system 19 which is connected to a fitting 21 threaded into thebottom or base plate 14 and which is in communication with the chamber18.

A cylindrical enclosure 22 formed of a suitable material such as sheetmetal is mounted on the top side of the top plate 13. The enclosure isprovided with a cylindrical side wall 23 and a planar top wall 24. Theside wall 23 is provided with an inturned bottom portion or lip 23awhich can be secured to the top plate 13 by suitable means such asscrews (not shown). Means is provided for supplying a gas to the chamber18 and consists of a gas supply 26 in the form of Argon or othersuitable gas which is supplied through a control valve 27 to a pipe 28that extends through the side wall 23 of the enclosure 22 and thenceinto a fitting 29 threaded into the top plate 13 and opening into thechamber 18.

Supporting means 31 is provided within the chamber 18 for supporting anarticle to be coated. The supporting means consists of a substrateholder 32 which is provided with an upper planar horizontal surface 33.The holder 32 is supported by a pipe or tube 34 which extends through aboss 36 provided on the bottom or base plate 14. O-rings 37 are providedfor establishing an air-tight and are in communication with the space(not shown) provided within the substrate holder 32. As can be seen, thetube 38 is provided as an inlet for the water, whereas the tube 39 isprovided as an outlet for the water. In certain applications ashereinafter described, the tube 38 can also be utilized as a conductorfor supplying RF to the substrate holder.

The substrate holder 32 can be formed of a suitable material such asaluminum. However, to prevent corrosion from the water, the innersurface exposed to the water can be plated with copper. The tubes 38 and39 can be formed of copper. It is apparent that, if desired, the tubes38 and 39 could be in the form of coaxial tubes. The tube 34 can beformed of a suitable material such as stainless steel.

The substrate which is to be coated is placed upon the substrate holder32. Thus, there has been provided a substrate 41 as shown in FIGS. 1 and2 which is adapted to be coated as hereinafter described.

The top plate 13 is provided with a hole or opening 46 which isgenerally opposite the substrate supporting means and in particular thesubstrate holder 32. This hole or opening 46 is defined by an annularvertical side wall 47. The top plate 13 is also provided with an annularrecess 48 which has a diameter slightly larger than the diameter of theopening 46 as shown in FIG. 1.

A target electrode assembly 51 is mounted in the hole or opening 46opposite the substrate holder 32 and consists of a target electrode orplate 52. The target electrode can be formed of any suitable materialsuch as aluminum. The target electrode 52 is generally circular as shownand is provided with a bottom or innermost planar generally horizontalsurface 53. It is also provided with an outer annular side surface 54which is at right angles to the surface 53. The target electrode alsohas a top planar surface 56 which is parallel to the surface 53. Thetarget electrode is also provided with an upstanding annular rib 57 anda centrally disposed upstanding boss 58. An annular recess 59 is formedby the rib 57 and the boss 58.

The target electrode assembly 51 includes means for supporting thetarget electrode 52 in the opening 46 and for insulating the same fromthe top plate 13. This means consists of an insulating support member 61formed of a suitable insulating material such as Delrin. The supportmember 61 is of such a size so that it can fit into the annular recess48. The insulated support member 61 is provided with a centrallydisposed opening 63 and has an O-ring 64 mounted in an annular recess 66which opens sidewise into the opening 63. The insulating support member61 is adapted to fit over the outer side of the annular rib 57 and theO-ring 64 is adapted to form a seal with respect to the same.

The insulating support member 61 is also provided with another O-ring 67disposed in an annular recess 68 which is adapted to establish a sealingengagement with the top plate 13. The support member 61 is provided witha deep annular slot 71 which opens through the bottom side as shown inFIGS. 1 and 2 and which is provided for a purpose hereinafter described.The support member 61 is also provided with a relatively shallow wideannular recess 72.

The target electrode assembly 51 includes a cap 76 formed of a suitablematerial such as aluminum. The cap is provided with a centrally disposedboss 77 which is of such a size so that it is adapted to fit within therecess 59. The cap 76 is also provided with a bore 78 which is adaptedto accommodate the boss 58 as shown in 'FIG. 2. Suitable sealing meansis provided in the form of O-rings 79 and 81 disposed in annularrecesses 82 and 83. A retaining screw 86 extends through a hole 87provided in the cap 76 and is threaded into the boss 58 of the targetelectrode 52. A thumb-nut 88 is threaded onto the screw 86 and isprovided for pulling the cap 76 downwardly toward the target electrodeand to thereby clamp the insulated support member 61 between the targetelectrode and the cap 76.

When the target electrode assembly 51 is in place, it

can be seen that the side surface or edge 54 of the target electrode isspaced a relatively small distance from the side wall 47 of the topplate 13. For reasons well known to those skilled in the art, this spaceshould be less than one Debye length. In addition, it can be seen thatthe surface 53 is generally flush with the innermost surface of the topplate 13.

The cap 76 is provided with a pair of additional holes 91 extendingthrough the cap and which are in communication with the recess 59.Fittings 92 are threaded into the holes 91 and are adapted to beconnected to a water supply in such a manner that one of the fittings isconnected to a pipe or tube 93 which supplies water to the recess 59 andthe other is connected to a pipe or tube 94 which takes water out of therecess 59. Thus, it can be seen that means has been provided for watercooling the target electrode 52.

Means is provided for supplying electrical energy to the targetelectrode 52 and consists of a cable 96 which is attached to the cap 76by a screw 97. The cable 96 is connected to an appropriate matchingnetwork 98 which, in turn, is connected to a suitable RF power supply99.

The target electrode assembly 51 also includes a dark space shieldassembly 101. The dark space shield assembly 101 consists of a pluralityof flat rings 102 which are spaced apart by spacers 103 and which aresecured to the bottom side of the top plate 13 by screws 104. The darkspace shield assembly 101 also includes an annular skirt 106 which isL-shaped in cross-section and which extends downwardly from the topplate adjacent the outer extremities of the rings 102. It should benoted that the inner extremities of the rings 102 are generally in linewith the annular side wall 47 forming the hole or opening 46. It alsoshould be noted that the target electrode 52 has substantially the samediameter as the diameter of the surface 33 of the substrate holder 32.

Means is provided for shielding the substrate holder 32 from the targetelectrode 52 and consists of a shutter 1'11 which is carried by an arm1-12. The arm 112 is secured to a shaft 113 which is rotatably mountedin bushing 114 provided in the top plate 13'. The shaft 113 is adaptedto be rotated manually or, if desired, a rotary solenoid or othersuitable means can be provided for rotating the same.

Operation and use of the sputtering system utilizing the targetelectrode assembly incorporating the present nvention may now be brieflydescribed as follows. Let it be assumed that the top plate 13 has beenremoved and that a substrate such as semiconductor substrate 41 has beenprovided upon the substrate holder 32 and that it is desired to coat thesame. Also, let it be assumed that the target electrode 52 has beenselected so that the appropriate material will be deposited upon thesubstrate 41. A vacuum such as 2x 10 torr is applied to the chamber .18.Electrical energy in the form of RF power is then supplied to the targetelectrode to cause sputtering to take place. Sputtering is a well knownphenomenon in which atoms from the target electrode 52 are driven off orsputtered by bombarding ions. In this process, the momentum of thebombarding particles is more important than their energy. For example, ahydrogen or helium ion accelerated to 3000 ev. will cause very littlesputtering compared to an ion of argon (which is chemically inert) withthe same 3000 ev. energy simply because much light hydrogen or heliumion carries very little momentum.

The sputtered atoms travel until they strike the substrate 41 where theydeposit to form the desired layer. As individual atoms, they arechemically active and readily form compounds with the atoms of thebombarding gas. This is why inert argon is used as a bombarding gas. Insome applications, however, a reactive gas is purposely added to argonso that the deposit is a film of a chemical compound rather than of thetarget material.

When the argon ions strike the target, the electrical charge isneutralized and the ions return to the process as atoms. If the targetis an insulator, the neutralization process results in a positive chargeon the target surface. This charge may grow to the point where thebombarding ions are repelled and the sputtering process stops. Tocontinue the process, the polarity must be reversed to attract enoughelectrons from the discharge to eliminate the surface charge. Thisperiodic reversal of polarity is done automatically by applying -RFvoltage onto the target electrode assembly (hence the term RFsputtering). The target and discharge system have a dioderectifier-likebehavior. This is caused by the vast difference in mobility of ions andelectrons. The electrons, being so much lighter, are attracted in muchgreater numbers to the target during the positive half period of the RFvoltage than are the heavy ions during their half period.

Various types of sputtering can be utilized with a sputtering system ofthe type herein described. In the sputter deposit mode, all the RF poweris applied to the target electrode, i.e. only the target is bombarded byions and sputtered. The substrate support or table is grounded.

In the bias sputter mode, RF power is applied to the I target but asmall amount of RF power is also applied to the substrate support ortable. Consequently, the substrates on the support or table are alsobombarded by ions but to a much smaller degree than the target.

In the sputter etch mode, all RF power is applied to the substratesupport or table and material is removed (etched away) from thesubstrate by ion bombardment. In the DC. bias mode, all of the RF poweris applied to the target but in place of grounding the substrate supportor table, a DC. bias is applied to the substrate table from an externalDC. power source.

Target materials can be bonded onto the target electrode or cathodeusing various techniques such as brazing, soldering or epoxy bonding.

It has been found that the sputtering system is particularly effectivefor all types of sputtering operations. This is primarily due to thegreatly reduced electrical capacitance between the target electrode 52.and the grounded parts of the housing 11 and the enclosure 22. It hasbeen found that this electrical capacitance has been reduced by a factorof approximately 2:1. By way of example, prior art equipment had acapacitance of approximately 130 picofarads with a 6 inch target,whereas the present system with a 6 inch target has a capacitanceranging from 70 to 80 picofarads. This 70 to 80 picofarad represents aresidual electrical capacitance between the edge of the target 52 andthe top plate 13 and the dark space shield assembly 101. The electricalcapacitance has been substantially reduced because the back side of thetarget electrode now faces the insulated support member 61. Only theinnermost surface 53- and the side surface 54 are exposed to theinterior of the chamber 18 and the spaces therein. The closest groundedparts to the back side of this target electrode 52 are the top wall 24of the enclosure 22 which in a typical installation could be at least 6or 7 inches above the target electrode.

The dark space shield assembly 101 serves two basic purposes. It focusesthe discharge and provides grounding surfaces for the discharge. It alsoprevents excessive cross-contamination of targets by sputtered materialwhere more than one target is utilized. The mounting of the targetelectrode is such that the back side and the side edges are shielded sothat sputtering will only occur from the front side.

The construction of the target electrode 51 is relatively simple andmakes it possible to readily remove the target electrode assembly sothat different target electrodes 52 can be provided or so that thetarget electrode can be covered with the appropriate material. Thetarget electrode assembly 51 is also constructed in such a manner sothat a good seal is formed with the top plate. For this purpose, it hasbeen found that the Delrin forms an excellent material for the supportmember 61 particularly when it is provided with the annular slot 71which permits the Delrin to flex and to thereby make it possible toobtain a better seal with the top plate. As shown, it is desirable toprovide the annular recesses for the O-rings within the Delrin becausethe Delrin is easy to machine. The slot 77 is also advantageous in thatit prevents electrical shorts due to spurious deposition on theinsulated support member 61.

It is apparent from the foregoing that there has been provided a new andimproved sputtering system and in particular a target electrode assemblyfor use therewith in which greatly reduced electrical capacitance toground has been obtained to thereby eliminate excessive circulatingcurrents. Both the target electrode 52 and the substrate holder arewater-cooled. In addition, the construction of the target electrodeassembly is such that an excellent seal is obtained between the targetelectrode assembly and the top plate. The surface of the targetelectrode from which sputtering occurs is the only significant surfacewhich is exposed to the inert gas utilized.

We claim:

.1. In a sputtering system, a housing forming an enclosed chamber, meansfor supplying a vacuum to the chamber in the housing, supporting meanswithin the chamber for supporting an article to be coated, said housingincluding a metallic plate having an opening therein generally oppositethe supporting means, a metallic target electrode having a front sideand a back side and an insulating member secured to said targetelectrode and mounted on said plate to support said target electrode insaid opening in said plate with the front side facing the supportingmeans and with a space surrounding said target electrode between theedge of the target electrode and the plate, said insulating member beingspaced from and directly facing the back side of the target electrodeand having a size which is larger than the back side of the targetelectrode.

2. A system as in claim 1 wherein the innermost surface of said targetelectrode is flush with the innermost surface of the plate on which itis mounted.

3. A sputtering system as in claim 1 together with a dark space shieldassembly secured to the bottom side of said plate and generallysurrounding said target electrode.

4. A system as in claim 3 wherein said dark space shield assemblyinclude a plurality of spaced rings surrounding the space underlying thetarget electrode.

5. A system as in claim 1 wherein said target electrode assemblyincludes a cap and means securing said cap to said target electrodewhereby said insulated support member is clamped between said targetelectrode and said cap.

6. A system as in claim 1 together with means for supplying a coolingliquid to the target electrode.

7. A system as in claim 1 wherein said insulated support member isprovided with a deep annular slot to permit said insulated supportmember to flex.

8. A system as in claim 7 wherein said slot has a greater radius thansaid target electrode.

9. In a target electrode assembly for use with a sputtering system, atarget electrode having a planar surface, an insulating member spacedfrom and mounted on said target electrode and having a size greater thanthe size of the target electrode, a cap extending over said insulatedsupport member, and means for securing said cap to said target electrodeto clamp said insulated support member between the same.

10. An assembly as in claim 9 together with means for establishing anair-tight seal between the insulated support member and the targetelectrode.

11. An assembly as in claim 9 wherein said target electrode is providedwith an upstanding rib together with means for forming an air-tight sealbetween said insulated support member and said cap.

12. A system as in claim 1 wherein said insulating member has an innersurface parallel to said back side of said target electrode and whereinthere is a space between said inner surface of said insulating memberand said target electrode.

13. A system as in claim 1 wherein said insulating member extends overthe space between the edge of the target electrode and the plate.

14. In a target electrode assembly for use with a sputtering system, atarget electrode having a planar surface and insulating member securedto said target electrode on the side of said target electrode oppositesaid planar surface, said insulating member extending beyond the outerperiphery of the target electrode, at least the outer margin of the sideof said target electrode facing the insulating member being spaced fromsaid insulating member and means extending through said insulatingmember for making electrical contact to said target electrode.

15. A target electrode assembly as in claim 14 wherein the outer marginof the side of said target electrode opposite said planar surface isprovided with a planar surface parallel to said first named planarsurface and wherein said insulating member is provided with a surfacewhich is spaced from said additional planar surface and is substantiallyparallel to said additional planar surface.

16. An assembly as in claim 15 together with a metallic cap extendingover said insulating member on the side opposite said target electrode,said cap having a planar surface parallel to said additional planarsurface of said target electrode.

References Cited UNITED STATES PATENTS 3,479,269 11/1969 Byrnes et al.204192 3,528,906 9/1970 Cash et al 204-298 3,595,775 7/ 1971 Grantham eta1 204-298 3,649,512 3/1972 Ackley 204-298 3,681,227 8/1972 Szupillo204-298 3,278,407 10/1966 Kay 204192 3,630,881 12/1971 Lester et a1204-298 JOHN H. MACK, Primary Examiner S. S. KANTER, Assistant Examiner

